But Labour, which is also losing part of its support base to Ukip, has only gone as far as saying it would hold a referendum if more powers were transferred from Westminster to Brussels.

"We support a referendum because we believe in democracy," Natalie Bennett, leader of the Green Party, told IBTimes UK in a wide-ranging interview.

"We trust in the voters. You have to be in your late fifties to have had any chance to vote on Europe. We wouldn't have necessarily started calling for it, but now it's on the table we say fine. Let's trust the voters."

Natalie BennettYouTube/Andrew Marr Show

Bennett said she would urge Labour leader Ed Miliband to commit to an in/out referendum if his party wins the election: "But I think given Miliband's track record on most things, continuing to sit on the fence is the most likely Labour position."

There has been disquiet for years in British politics about the EU, which critics say is undemocratic and erodes national sovereignty.

Proponents argue it is a huge market of 500 million citizens and so the British economy is better off by being inside where it can influence the rules and trade freely.

One of the fundamental pillars of the EU is the free movement of people.

Since the early 2000s as the 28-member EU incorporated poorer central and eastern European states, the UK has experienced mass immigration.

Many Britons perceive themselves as worse off as a result. They believe they face more competition for jobs, housing, schools and healthcare.